When
using a TDM to send Async data, there is no error correction on
anything sent, so if there are errors on the digital service errors
will occur on the multiplexers.
If
data is very critical, then an error-correcting statistical multiplexer
such as the Esprit should be used to ensure all data arrives correctly.
Problems
with Continuous Asynchronous Data
A
problem can occur if a TDM is being used to transport continuous
async data due to independent clock references being used within
connected equipment. Eloquence TDM multiplexers use a divisor
of the composite LINK clock to produce exactly the baud rate required
(referenced to the link clock) and uses this to output all the
characters sent over the link.
If
a TDM multiplexer is being sent continuous async characters at
a slightly faster baud rate (perfectly possible, as all async
devices use their own clock which is not referenced to anything
else and will be either slightly faster or slower than nominal),
then all the characters will be sent across the link perfectly,
but they will be arriving at the remote multiplexer SLIGHTLY FASTER
than they are being output. Eventually, the buffers at the remote
site will overflow, and errors will occur.
This
will, of course, even occur when the remote location has a loopback
connector fitted to it, or when it is set to loopback. It is simply
a regularly observed effect of async data when continuous characters
are sent (RS232/V.24/ASYNC was never designed/intended to support
continuous data).
If
any gaps occur in the data flow (which is a natural requirement
of async transmission), the buffers will empty out during these
gaps, so overflow will never occur. |
